Complete and limited substitution of rare-earth elements in apatite silicates La(9-x)Lnx(SiO4)6O1.5

• Published in: Solid state sciences Vol. 76; pp. 8 - 14
• Main Authors: Get'man, E.I., Ignatov, A.V., Prisedsky, V.V., Borysova, K.V.
• Format: Journal Article
• Language: English
• Published: Elsevier Masson SAS 01-02-2018
• Subjects: Apatite silicate; Isomorphism; Mixing energy; Powder XRD; Rare earth; Solid solution; Isomorphism; Powder XRD; Mixing energy; Rare earth; Solid solution; Apatite silicate
• ISSN: 1293-2558; 1873-3085
• DOI: 10.1016/j.solidstatesciences.2017.12.001

The isomorphous substitution of smaller RE elements (Ln = Nd, Eu, Gd, Ho, Tm, and Yb) for lanthanum in the apatite silicate solid solutions La9−xLnx(SiO4)6O1.5 was studied by X-ray powder diffraction and the Rietveld structure refinement, scanning electron microscopy and energy-dispersive X-ray microanalysis. Single-phase samples were prepared by solid-state synthesis at a moderate temperature of 1200 °C using an amorphous SiO2 nanopowder (10–40 nm) as a reactant. As the atomic number of Ln increases, the complete solubility, 0 ≤ x ≤ 9, found in the systems with Ln = Nd, Eu, Gd, and Ho changes to a limited one for Ln = Tm (0 ≤ x < 1.5) and Yb (0 ≤ x < 1). The distribution of La and smaller Ln over two structurally independent cationic sites is close to statistical. In both cationic polyhedra, Ln(1)O9 and Ln(2)O7, the bond lengths Ln – O decrease with x, except the longest bonds Ln(1) – O(3) and Ln(2) – O(1) which increase slightly. The experimental results on the substitution limits agree with the values of the mixing energy, and critical temperature of miscibility calculated in the approximation of a regular solid solution.
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•RE apatite silicates were synthesized at a moderate temperature 1200 °C.•Substitution limits in La9−xLnx(SiO4)6O1.5 were found for Nd, Eu, Gd, Ho, Tm, Yb.•La and smaller Ln are distributed statistically over two independent cation sites.•The bond lengths in cationic polyhedra decrease with x, except the longest bonds.•Found substitution limits agree with the values calculated for a regular solution.